Many electronic devices have audio amplifiers for generating an audio driving signal for an output transducer, e.g. a loudspeaker, which may for example be an on-board output transducer of the electronic device itself.
FIG. 1a illustrates one example of an amplifier circuit 100 as may be implemented in a battery powered device, for example a portable device such as a mobile telephone or smartphone or tablet computer or the like. A battery 101, in use, provides a power source, at least during some use cases where other sources of power are not available. Power from the battery 101 may typically, at least in some instances, be regulated by a voltage regulator, such as a DC-DC converter 102 which may, for instance, be a boost regulator as would be understood by one skilled in the art. As such a DC-DC converter 102 is a switched power supply, an output capacitor 103 is provided to maintain the voltage at the output of the DC-DC converter 102 across each switching cycle. This voltage maintained at the output of the DC-DC converter 102 is provided as a supply voltage VS to the amplifier 104.
The amplifier 104 may, in some instances be a Class-D amplifier, as Class-D amplifiers are generally quite power efficient, and power efficiency is typically an important consideration for portable electronic devices. However other types of amplifier could be used in some implementations.
The amplifier 104 receives an input signal SIN (which for a Class-D amplifier may be a digital signal) and amplifies the signal to provide a driving signal for driving a load 105, for instance a loudspeaker of the host device (i.e. the device of which the amplifier circuit 100 forms a part).
As will be understood by one skilled in the art the supply voltage VS provided to the amplifier should be sufficient for the amplifier to be able to amplify the input signal without clipping, allowing headroom for amplifier output stage voltage compliance and DC-DC converter output voltage ripple, at least within an expected range of input signal amplitudes.
With a battery powered device however there may be a battery current limit specifying the maximum current that should be drawn, in use, from the battery 101. Typically therefore the DC-DC converter 102 may have an input current limit which is set so as to ensure that the input current to the DC-DC converter 102 does not exceed the battery current limit.
This input current limit effectively limits the power that the DC-DC converter 102 can output and hence correspondingly the audio power that can be output from the amplifier 104.
Increasingly the audio performance specifications for some electronic devices may be such that a DC-DC converter 102 may be unable to meet the power demand of the amplifier 104 at peak levels of audio signal SIN without exceeding the battery current limit for practical batteries 101 that may be used in such electronic devices. Thus, to avoid unwanted signal clipping without exceeding the battery current limit may require the peak signal level achieved in use to be reduced to below that which would be desired.
Embodiments of the present disclosure are thus directed at methods and apparatus for signal amplification that at least mitigate at least some of the above mentioned issues.
Thus according to the present invention there is provided an amplifier circuit for amplifying an audio signal, comprising:                a voltage regulator having an input node for receiving an input voltage and an output node for outputting an output voltage, the voltage regulator comprising an output capacitor coupled to the output node;        an amplifier connected to receive the output voltage from said output node of the voltage regulator as a supply voltage;        the voltage regulator being operable in a voltage-control mode to maintain the output voltage at a nominal output voltage and in current-control mode to apply current limiting so as to limit an input current drawn by the voltage regulator via the input node so as not to exceed a defined input current limit; and        a controller for defining the nominal output voltage;        wherein the controller is operable in a first mode to define the nominal output voltage so as not to exceed a first voltage magnitude and is operable in a second mode to define the nominal output voltage to be equal to a second voltage magnitude, the second voltage magnitude being greater than the first voltage magnitude;        wherein the controller is configured to monitor an indication of the level of the audio signal for a high-amplitude part of the audio signal that could result in the voltage regulator operating in the current-control mode to apply current limiting; and        wherein the voltage controller is configured to operate in said first mode until such a high-amplitude part of the audio signal is detected and, on such detection to operate in the second mode of operation until such a high-amplitude part of the audio signal has been amplified;        wherein the second voltage magnitude is greater than required for voltage headroom for amplifying the high-amplitude part of the audio signal so as to allow for a voltage droop of the output voltage over a plurality of switching cycles of the voltage regulator when operating in the current-control mode.        
On detection of a high-amplitude part of the audio signal, the controller may be configured to change to the second mode of operation such that the output capacitor is substantially charged to the second voltage magnitude before the high-amplitude part of the audio signal is amplified by the amplifier.
In some embodiments the controller is configured to detect a high-amplitude part of the audio signal by determining when the indication of the level of the audio signal exceeds a first threshold corresponding to a signal level that would be expected to lead to the voltage regulator operating in the current-control mode to applying current limiting. The circuit may comprise a threshold generator for determining the first threshold. The threshold generator may be configured to determine the first threshold based on the defined input current limit. In some instances the defined input current limit may be configurable and, for instance, may be configured to vary with a battery voltage at the input node. The threshold generator may be configured to determine said the threshold based on at least one of: voltage regulator efficiency; amplifier efficiency; audio load impedance; audio load inductance; a minimum audio frequency; and amplifier gain. The controller may comprise a comparator for comparing the indication of the level of the audio signal with the first threshold.
In some instance the second voltage magnitude is greater than required for voltage headroom for amplifying the high-amplitude part of the audio signal so as to allow for a voltage droop of the supply voltage of at least 10% or at least 15%.
The controller may be configured such that in the first mode of operation the nominal output voltage provides a nominal headroom of 15% or less, or 10% or less, of the magnitude of the nominal output voltage. The nominal headroom is the voltage difference between the nominal output voltage and the maximum output voltage of the amplifier, in this case the maximum output voltage of the amplifier in the first mode of operation.
The controller may be configured such that in said second mode of operation the nominal output voltage provides a nominal headroom of at least 20%, or at least 25%, of the magnitude of the nominal output voltage. The nominal headroom, in this case, is the voltage difference between the nominal output voltage and the maximum output voltage of the amplifier in the second mode of operation.
The controller may be configured to receive a version of the audio signal and to determine the indication of the level of the audio signal. The amplifier circuit may comprise an envelope detector for receiving the version of the audio signal and determining an envelope value as the indication of the level of the audio signal.
In some embodiments there is a delay in a signal path for the audio signal upstream of the amplifier. The delay may comprise a buffer of configurable size. The delay may have a delay duration which is at least equal to the time that would be required for the output capacitor to be charged to from a lowest expected voltage level in the first mode of operation to the second voltage magnitude.
In some implementations, in the first mode of operation, the controller is configured to vary the nominal output voltage of the voltage regulator based on the indication of the level of the audio signal.
In some implementations, in the first mode of operation, the controller may be operable to control the voltage regulator in a pass-through mode to pass the input voltage directly to the output node as the supply voltage if the indication of the level of the audio signal is below a second threshold.
The voltage regulator may comprises a DC-DC converter. The controller may be configured to vary a voltage reference signal supplied to the DC-DC converter to define the nominal output voltage.
The amplifier circuit may be implemented as an integrated circuit. The amplifier circuit may, in use, comprise a loudspeaker configured to be driven by the amplifier. In some implementations there may be a battery and the input node of the voltage regulator may be coupled to receive the input voltage from said battery. The defined input current limit may correspond to a battery current limit.
Aspects relate to electronic device comprising an amplifier circuit as described in any of the variants discussed above. The electronic device may be at least one of: a portable device; a battery operated device; a communication device; a mobile or cellular telephone device; a smartphone; a computing device; a laptop, notebook or tablet computing device; a gaming device; a personal media player; a wearable device.
In another aspect there is provided an amplifier circuit for amplifying an audio signal, comprising:                an amplifier;        a voltage regulator having an input node for receiving an input voltage and an output node for outputting a supply voltage to said amplifier, the voltage regulator comprising an output capacitor coupled to the output node;        the voltage regulator comprising a voltage control loop for controlling the voltage at the output node to a nominal output voltage and a current control loop for, when activated, limiting an input current drawn by the voltage regulator via the input node so as not to exceed a defined input current limit, wherein the voltage regulator operates according to the voltage control loop unless the current control loop is activated; and a controller for defining the nominal output voltage;        wherein the controller is operable in a first mode to define the nominal output voltage so as not to exceed a first voltage magnitude and is operable in a second mode to define the nominal output voltage to be equal to a second voltage magnitude, the second voltage magnitude being greater than the first voltage magnitude;        wherein the controller is configured to monitor an indication of the level of the audio signal for a high-amplitude part of the audio signal that could result in the current control loop of the voltage regulator being activated; and        wherein the controller is configured to operate in said first mode until such a high-amplitude part of the audio signal is detected and, on such detection to operate in the second mode of operation until such a high-amplitude part of the audio signal has been amplified;        wherein the second voltage magnitude is greater than required for voltage headroom for amplifying the high-amplitude part of the audio signal so as to allow for a voltage droop of the supply voltage over a plurality of switching cycles of the current control loop dominating the voltage control loop.        
In a further aspect there is an amplifier circuit for amplifying an audio signal, comprising:                an amplifier;        a voltage regulator having an input node for receiving an input voltage and an output node for outputting a supply voltage to said amplifier, the voltage regulator comprising an output capacitor coupled to the output node;        the voltage regulator being operable to apply current limiting so as to limit an input current drawn by the voltage regulator via the input node so as not to exceed a defined input current limit;        a controller for defining a nominal output voltage generated by the voltage regulator;        wherein the controller is operable to compare an indication of the level of the audio signal to a first threshold, wherein the first threshold is indicative of the input signal that, if amplified by the amplifier, would be expected to lead to the voltage regulator experiencing current limiting; and        in the event that the indication of the level of the audio signal exceeds said first threshold to increase the magnitude of the nominal output voltage of the voltage regulator from a first operating range to a second higher voltage level.        
A yet further aspect provides an amplifier circuit for amplifying an audio signal, comprising:                an amplifier;        a voltage regulator having an input node for receiving an input voltage and an output node for outputting a supply voltage to said amplifier, the voltage regulator comprising an output capacitor coupled to the output node;        the voltage regulator being operable to apply current limiting so as to limit an input current drawn by the voltage regulator via the input node so as not to exceed a defined input current limit;        a threshold generator for generating a first threshold indicative of an audio signal level that would be expected to lead to current limiting being applied by the voltage regulator; and        a controller for controllably varying a nominal output voltage generated by the voltage regulator;        wherein the controller is operable in a first mode to control the nominal output voltage so as not to exceed a first voltage magnitude and in a second mode to control the nominal output voltage to be equal to a second voltage magnitude, the second voltage magnitude being greater than the first voltage magnitude;        wherein the controller operates in the first mode unless and until the indication of the level of the audio signal exceeds the first threshold and then changes to the second mode of operation until a predetermined time after the level of the audio signal has dropped back below the first threshold.        
In another aspect there is an amplifier circuit for amplifying an audio signal, comprising:                an amplifier;        a voltage regulator having an input node for receiving an input voltage and an output node for outputting a supply voltage to said amplifier, the voltage regulator comprising an output capacitor coupled to the output node;        the voltage regulator being operable to apply current limiting so as to limit an input current drawn by the voltage regulator via the input node so as not to exceed a defined input current limit; and        a controller for controlling a nominal output voltage generated by the voltage regulator;wherein the controller is operable in a first mode to control the nominal output voltage so as not to exceed a first voltage magnitude and is operable in a second mode to control the nominal output voltage to be equal to a second voltage magnitude, the second voltage magnitude being greater than the first voltage magnitude;        wherein the controller is configured to monitor an indication of the level of the audio signal for a high-amplitude part of the audio signal that could result in the voltage regulator applying current limiting; and        wherein the controller is configured to operate in said first mode until such a high-amplitude part of the audio signal is detected and, on such detection to operate in the second mode of operation until such a high-amplitude part of the audio signal has been amplified;        wherein the second voltage magnitude is greater than required for standard voltage headroom for amplifying the high-amplitude part of the audio signal.        
In another aspect there is an amplifier circuit for amplifying an audio signal, comprising:                an amplifier;        a voltage regulator having an input node for receiving an input voltage and an output node for outputting a supply voltage to said amplifier, the voltage regulator comprising an output capacitor coupled to the output node;        the voltage regulator being operable to apply current limiting so as to limit an input current drawn by the voltage regulator via the input node so as not to exceed a defined input current limit; and        a controller for controlling a nominal output voltage generated by the voltage regulator;        wherein the controller is configured to monitor an indication of the level of the audio signal for a high-amplitude part of the audio signal above a first threshold; and        wherein the controller is configured to operate in a first mode until such a high-amplitude part of the audio signal is detected and, on such detection to operate in the second mode of operation until such a high-amplitude part of the audio signal has been amplified;        wherein in the first mode of operation the nominal output voltage provides a nominal voltage headroom of 10% or less of the magnitude of the nominal output voltage; and        wherein in the second mode of operation the nominal output voltage provides a nominal voltage headroom of at least 25% of the magnitude of the nominal output voltage.        